In this paper, a compact multiple input multiple output (MIMO) antenna with the size of 44 mm × 27 mm × 0.8 mm is designed for operation in ultra-wideband (UWB) frequency range, i.e., from 3.1 GHz to 10.6 GHz. The design consists of two parallel and symmetric radiators fed with coplanar waveguide (CPW) and printed on the same side of a substrate. To enhance isolation and increase impedance bandwidth, a T-shaped slot is etched out of the ground plane. Simulations are employed to design the two antenna elements and to study their performance in terms of reflection coefficient, mutual coupling, current distribution, radiation pattern, and envelope correlation coefficient (ECC) for pattern diversity. Simulated and measured results show that the impedance bandwidth of the proposed antenna covers the whole UWB frequency range with an isolation of better than −15 dB between the two elements. Measured results show that the proposed antenna is a good candidate for portable MIMO/diversity UWB applications.